1. Field of the Invention
The present invention relates to dental cements and, in particular, to dental glass ionomer cements. More specifically, the present invention is concerned with fluoroaluminosilicate glass powders designed for use as dental glass ionomer cements.
2. Statement of the Prior Art
Dental glass ionomer cements are basically obtained by allowing an acid component composed mainly of polyacrylic acid to react with fluoroaluminosilicate glass powders in the presence of water for setting, and are one of the materials which have been widely used in the dental field. With the glass ionomer cements characterized in that they are of very excellent biocompatibility, show chemical bond to tooth substance and provide a hardened mass which is translucent and of excellent estheticity, they are further expected to reinforce or strengthen tooth due to the fluorine contained therein. Because of their outstanding advantages the glass ionomer cements have been widely used for many purposes, like restorative filling, cementation of crowns, inlays and bridges or orthodontic bands, lining of cavities, core build-up and pits and fissure sealing.
However, only a combination of aluminosilicate glass powders with polyacrylic acid produces cement paste, poor in both flow and working property. In addition, since the cement requires a long time for complete set, it disintegrates on the surface when contact with saliva or water in a patient mouth and becomes brittle, thus failing to produce its final strength and estheticity. As well-known in the previous report, numerous methods have been investigated so as to overcome these disadvantages. For instance, Japanese Patent Laid-Open No. 52(1977)-101893 for public inspection discloses a setting liquid containing a 45 to 60% aqueous solution of polyacrylic acid or acrylic copolymer and one or more of polybasic carboxylic acids in an amount 7 to 25% of the total weight. With this liquid, higher reaction rates and improved crushing strength were achievable. Japanese Patent Laid-Open No. 57(1982)-2210 for public inspection discloses a setting liquid for glass ionomer cements which contains tartaric acid and a fluoro complex salt with an acrylic acid copolymer, and is found to produce the aforesaid effects and result in less solubility. On the other hand, Japanese Patent Application No. 60(1985)-206299 discloses dental glass ionomer cements containing glass powders treated on the surfaces with a fluoride, and shows that a cement paste just after mixing is improved in its flow as well as in mixing property.
As stated in the foregoing, various investigations have been made on improvements in dental glass ionomer cements, but the dental cements developed by now are foound to have many disadvantages and to be far from the ideal. For instance, while the solubility of dental zinc phosphate cements to distilled water is about 0.03%, as measured according to JIS T 6602, that of glass ionomer cements are 0.2% and more. In this respect, there is left much to be modified. When glass ionomer cements are actually applied in the mouth for clinical purposes, reduction of their solubility seems indispensable for their better durability in the mouth. In the case of current glass ionomer cements, a cement-surface embrittling phenomenon, which is believed to correlate with the solubility, occurs when they contact with saliva in an earyl stage of the process of setting. The surface of cement, which is not completely hardened, is easily affected by water, and dissolve therein, thus offering a problem of losing its translucency. This problem leads to a grave defect in view of aestheticity, when that cement is used for restorative filling. For that reason, it is necessary to apply a varnish capable of being resistant to water on the surface of cement just after filling so as to shut off the influence of water. Further, hardened cements have had somewhat improved crushing strength, but have still been insufficient, compared with that of tooth substance. There is thus an increasing demand toward further improving its crushing strength.
Referring to the general properties of dental materials supplied to clinicians in the form of half-made products, working time should be preferably long and, contrary, setting time should be short. In other words, cement paste should preferably retain flowability for only the required time, and be set as sharply as possible. However, current glass ionomer cements are still far from ideal in this regard.